The vertebral column is a bio-mechanical arrangement composed largely of ligaments, muscles, vertebrae, and intervertebral discs. The bio-mechanical functions of the spine include: (1) support of the body, which involves the transfer of the weight and the bending movements of the head, trunk and arms to the pelvis and legs; (2) complex physiological motion between these parts; and (3) protection of the spinal cord and nerve roots.
As populations age, it is anticipated that there will be an increase in adverse spinal conditions which are characteristic of aging. For example, with aging comes an increase in the degeneration of the intervertebral disc. Disabling mechanical pain resulting from disc degeneration is often treated surgically with an interbody fusion.
The primary purpose of the intervertebral discs, located between the endplates of the adjacent vertebrae, is to distribute forces between vertebrae, stabilize the spine, and cushion vertebral bodies. Thus the intervertebral disc acts as a shock absorber for the spinal column. A normal intervertebral disc includes a semi-gelatinous component which is surrounded by an outer ring called the annulus fibrosus. In a healthy spine, the annulus fibrosus prevents the gelatinous component from protruding outside the disc space.
Spinal discs may be displaced or damaged as a result of disease, trauma, aging or injury to the spine. Frequently, the only relief from the disability caused by degenerated spinal discs is a discectomy, or surgical removal of the intervertebral disc followed by fusions of the adjacent vertebrae. The removal of the damaged or unhealthy disc without reconstruction would allow the disc space to collapse, resulting in further instability of the spine, abnormal joint mechanics, premature development of arthritis or nerve damage, in addition to severe pain. To prevent the intervertebral space from collapsing, a structure must be placed within the intervertebral space to provide support.
For example, in early spinal fusion techniques, bone material, or bone osteogenic fusion devices were simply placed between the transverse processes of adjacent vertebrae. The osteogenic fusion material consisted of cortical-cancellous bone which was not strong enough to support the weight of the spinal column at the instrumented level. Consequently, the spine was stabilized by way of a plate or a rod spanning the affected vertebrae.
For example, U.S. Pat. No. 4,604,995 assigned to Stephens, David C. and Morgan, Craig D., discloses “a surgical implant for imparting stability to the thoraco-lumbar spine by fixation of the implant to the spine with segmental spinal instrumentation, the implant comprising: a unitary rod having a generally rectangular configuration formed by a pair of spaced apart branches substantially mirror image duplicates of one another and substantially equally spaced apart along their entire length; a bight end piece interconnecting the branch pair at one end portion thereof; and a gate forming end piece connected to close the other end portion of the branch pair except for a small gate opening to provide access to the space between the branch pair.”
With this technique, once the fusion occurs, the hardware maintaining the stability of the spine becomes superfluous. There are other several disadvantages associated with the use of the abovementioned metal implants. Solid body metal implants do not effectively enable bone in-growth which may lead to the eventual failure of the implant. Surface porosity in such solid implants does not correct this problem because it will not allow sufficient in-growth to provide a solid bone mass strong enough to withstand the loads of the spine. Attention was then turned to implants, or interbody fusion devices, which could be interposed between the adjacent vertebrae, maintain the stability of the disc interspace, and still permit fusion or arthrodesis.
For example, U.S. Pat. No. 4,961,740 assigned to Centerpulse USA Inc., discloses “a fusion cage adapted for promoting fusion of one or more bone structures when bone-growth-inducing substance is packed into the fusion cage, comprising: a cage body defining a cavity with an inner surface; said cavity adapted to be packed with the bone-growth-inducing substance; said cage body defining an outer surface; means for defining threads on the outer surface of the cage body and adapted for biting into the bone structure; said threads defining means including a plurality of threads which define valleys there between; a plurality of perforations provided in the valleys of the threads for providing communication between the outer surface and the cavity in order to allow immediate contact between the one or more bone structures and the bone-growth-inducing substance packed into the fusion cage”.
U.S. Pat. No. 5,026,373 assigned to Surgical Dynamics, discloses “a method for surgically preparing two adjacent bony structures for implanting a hollow cylindrical fusion cage that has an external, substantially continuous helical thread which defines a plurality of turns with a valley between adjacent turns and that is perforated in the valley between adjacent turns of the thread, said method comprising the steps of: (a) drilling a pilot hole laterally between said bony structures, (b) inserting a pilot rod into the pilot hole, (c) fitting a hollow drill over the pilot rod, (d) with the hollow drill, enlarging said pilot hole to form a bore that penetrates into the cortical bone of each of said bony structures, and (e) tapping a female thread into the wall of said bore, the crown of which female thread penetrates into the cancellous portion of each of said bony structures, which female thread can mate with the helical thread of the fusion cage.”
The abovementioned intervertebral fusion device has substantial disadvantages, however. The metallic supporting frame of the prior art fusion cages is not osteoconductive and therefore does not form a strong mechanical attachment to a patient's bone tissue. This can lead to graft necrosis, poor fusion and poor stability. Moreover, many of these devices are difficult to machine and therefore expensive. Furthermore, the fusion cages may stress shield the bone graft, increasing the time required for fusion to occur. The abovementioned implants further requires a special tool and additional preparation of the adjacent vertebral bodies to ensure fusion.
In addition, the use of bone graft materials in the prior art metal cage fusion devices presents several disadvantages. Autografts, bone material surgically removed from the patient, are undesirable because the donor site may not yield a sufficient quantity of graft material. The additional surgery to extract the autograft also increases the risk of infection, persistent pain, and may reduce structural integrity at the donor site.
U.S. Pat. No. 5,489,308 assigned to Zimmer Spine, Inc., discloses “an implant for insertion into a bore formed between opposing vertebrae of a spine where said vertebrae are separated by a spacing with a disk material having an annulus disposed within said spacing, said implant comprising: a rigid body having a leading end and a trailing end spaced apart by a longitudinal axis of said body; said body comprising at least exposed threads disposed at least partially between said leading end and said trailing end; said threads selected to engage vertebra material and draw said body along a direction of said axis upon rotation of said body about said axis; said body having a hollow, generally cylindrical shell with said threads disposed on an exterior surface of said shell; said body having means defining a chamber disposed within said body and said body is provided with a rib disposed within said cylindrical shell and extending radially inwardly toward said longitudinal axis, said rib dividing said chamber into a leading end chamber and a trailing end chamber, and said rib including at least a rigid extension extending between and connecting diametrically opposed sides of said body; said body having means defining at least one opening formed through said body in communication with said chamber and with said opening extending generally radially to said axis; and said body having a transverse dimension generally transverse to said longitudinal axis and dimensioned so as to be greater than said bore for said body to urge said opposing vertebrae apart and to stretch said annulus upon insertion of said body into said bore between said vertebrae with a portion of said body opposing a first of said opposing vertebrae and with an opposite side of said body opposing a second of said opposing vertebrae.”
One problem with the implant devices of the type mentioned above is that they tend not to maintain the normal curvature of the spine. In a healthy state, the cervical and lumbar areas of the human spine curve convexly forward. Normal lordosis results, at least in significant measure, from the normal wedge-shaped nature of the spaces between adjacent pairs of the cervical and lumbar vertebrae, and the normal wedge-shaped nature of the intervertebral discs that fill these spaces. Loss of lordosis and proper intervertebral spacing may result in an increased risk of degeneration to other intervertebral discs located adjacent to the fusion level due to the alteration of the overall mechanics of the spine.
A further problem with the abovementioned implant is that the cylindrical geometry of the engaging element tends to provide a small area of contact between the engaging element and the vertebrae. The small engaging surface tends to contribute to subsidence or deformation of the cortical layer of the vertebrae adjacent to the engaging element. Moreover, the small engaging surface provides less contact between the bone graft material encased in the device and the adjacent vertebrae. Exposure of the bone graft material to the surface of the vertebrae is important because the greater the area of contact, the greater the possibility of having a successful fusion.
U.S. Pat. No. 6,143,033 discloses “an allogenic intervertebral implant for use when surgical fusion of vertebral bodies is indicated. The implant comprises an annular plug conforming in size and shape with the end plates of adjacent vertebrae and has a plurality of teeth positioned on the top and bottom surfaces for interlocking with the adjacent vertebrae. The teeth preferably have a pyramid shape or a saw-tooth shape.” The teeth 105 of a prior art implant are shown in FIG. 1.
U.S. Pat. No. 6,986,788 discloses “an allogenic intervertebral implant for use when surgical fusion of vertebral bodies is indicated. The implant comprises a piece of allogenic bone conforming in size and shape with a portion of an end plates of the vertebrae and has a wedge-shaped profile with a plurality of teeth located on top and bottom surfaces.” The teeth 205 of the implant 200 have a pyramidal shape, as shown in FIG. 2.
However, the implants are not sufficiently effective at preventing expulsion of the implant. The surfaces of the implants, whether pyramidal 205 or saw tooth 105, do not effectively provide implant stability.
In the light of the abovementioned disadvantages, there is a need for improved methods and systems that can provide effective, efficient and fast intervertebral fusion device. Specifically, an intervertebral implantable device is needed that conforms to the endplates of the patient's adjacent vertebrae, maintains the normal disc spacing, and appropriate curvature of the spine. Further, an approach is needed that maximizes the probability of success of bone fusion, provides instant stability to the spine while fusion occurs, is easily implantable, and minimizes trauma to the patient and the possibility of surgical and post-surgical complications.